1. Field of the Invention
The invention relates to systems for the reading of fingerprints used especially in devices for authenticating individuals.
2. Description of the Prior Art
The many systems used to authenticate individuals, based on fingerprint analysis, comprise at least one sensor used to obtain an image of the fingerprint of the individual to be identified. In present systems, the finger is placed on the sensor whose reading surface must necessarily have a size of the order of magnitude of the size of the finger. The sensor is associated with a system of analysis used to compare the image of the fingerprint that it gives with an image of a reference fingerprint stored in an adequate medium, for example a chip card.
In most cases, the sensors give an analog type of information element and the system of analysis makes use of an operation for the digital processing of the image of the fingerprint which must be digitized at output of the sensor by means of an analog-digital converter. In certain embodiments, the sensor delivers the digitized image directly.
Fingerprint reading systems are often based on the use of optical devices such as a video camera picking up the image of the finger but a simple photograph of the same finger can be used to obtain the same image at output of the camera and thus defraud the system. To overcome this drawback, certain systems use prisms or microprisms in order to ascertain that it is really a genuine finger and not a photograph that is being placed before the sensor, the light being reflected only at the places where the lines of the fingerprint do not touch the prism. A photograph then becomes inoperative. However, the optical systems cannot be used to ascertain that the finger that has been placed before the sensor is truly a live finger and is not for example a mold. The optical systems have other drawbacks such as for example their great volume and high production cost.
Other means have been proposed to make devices for the authentication of individuals by fingerprints, making use of the batch-processing possibilities of the semiconductor industry, which are therefore potentially less costly and provide advantages of the integration of the sensor and of all or a part of the data-processing sequence of the authentication device, especially the operations of image digitizing at output of the sensor, the storage of the reference image and authentication. The fingerprint-reading sensor has a matrix of sensitive elements organized in rows and columns, giving an electric signal that differs depending on whether a ridge of the fingerprint line touches or does not touch a sensitive element of the sensor.
Patents have been filed on various means of reading fingerprints:
the U.S. Pat. No. 4,353,056 describes a principle of reading based on the capacitance of the sensitive elements of the sensor.
Other systems comprise sensors having components sensitive to pressure, temperature or else to pressure and temperature converting the spatial information of pressure and/or temperature into an electric signal that is then collected by a semiconductor-based multiplexer which may for example be a CCD matrix. The U.S. Pat. No. 4,394,773 describes a principle of this kind.
The sensors based on the piezoelectric and/or pyroelectric effects are the most valuable for they are sensitive to pressure and/or to heat exerted on their sensitive elements. This feature makes it possible to ascertain, during the reading of fingerprints, that the finger is truly part of a living individual through the inherent heat that it releases. It is also possible to detect the variations due to the flow of blood in the finger, inducing a variation of heat and/or pressure, thus providing for greater reliability in the authentication of the fingerprint.
These types of sensors, which can be directly integrated into a semiconductor substrate, have drawbacks that hamper their entry into the market. The surface area of the sensor necessarily has the order of magnitude of the size of a finger, namely about several square centimeters to about ten square centimeters when it is desired to have the entire first phalanx of the finger which in this case has to be rolled on the sensor so as to have the entire fingerprint on the sensor. This reduces the number of individuals that can be authenticated by means of a silicon wafer. The efficiency of manufacture of silicon wafers diminishes in proportion to their surface area and thus considerably increases the cost of manufacture.
The electric signal given by the sensors integrated into a semiconductor substrate is fleeting and a specific system is necessary to maintain it in time for the electric charges are induced by variations of the physical effects (temperature, pressure, etc.) on the sensor. As a consequence, the signal at its output tends to disappear when the physical effects are balanced. The time constants of disappearance of the signal are in the range of some milliseconds to some seconds in favorable cases.
The practical result of this is that a series of images is produced starting from the time when the finger is placed on the sensor. The quality of contrast of these images is not stable and they tend to fade away. This complicates the task of the recognition system for it is then necessary to analyze all the images that are being constantly produced by the sensor in order to find the one most appropriate for authentication.
Systems with excitation external to the sensor have been proposed, for example the sending of an energy beam in the form of microwaves, but they complicate the system and increase its volume and cost.
It is possible to overcome the effects of the disappearance of the image of the fingerprint by means of an electronic memory. However this complicates the designing of the sensor and increases its cost of manufacture for it requires technology that enables memory storage. It is very difficult to build a system that is sufficiently precise, reliable and inexpensive, and capable of deciding which is the best image among those produced by the sensor.
The present invention proposes to overcome the drawbacks of the prior art by proposing a fingerprint-reading system comprising means for reading a fingerprint when the finger and a sensor belonging to the reading means are in contact and in a relative motion of sliding of the sensor and the finger with respect to each other and means to reconstitute an image of the fingerprint from partial images obtained during this motion.
A sliding of the finger on a sensor fixed to a frame or the sliding of a mobile sensor on a finger that is held stationary or more generally the sliding of the finger and of the sensor with respect to each other stabilizes the quality of the image given by the sensor. Indeed, when there is a sliding of the finger on the sensor, the physical variations at each sensitive element of the sensor are permanent for the lines of the fingerprint touch it successively with a speed that is in the same range as or faster than the time constant characteristic of the sensitive layer of the sensor. The sensor, under these conditions, provides a sequence of images with a constant quality of contrast.
Another aspect of this invention lies in the fact that, inasmuch as a relative sliding of the finger on the sensor is done, it is possible to reduce the size of the sensor to dimensions smaller than the size of the finger. For example, assuming that the finger shifts on the sensor in the direction of its length, the length of the sensor may be reduced and will no longer cover more than a small surface area of the fingerprint. In this case, the electric signals given by the sensor during a relative sliding of the finger on the sensor correspond to a succession of partial images of the fingerprint and inasmuch as the relative speed of shift of the finger with respect to the sensor does not exceed a certain maximum value, an image given by the sensor at a given instant will at least partially overlap the following one. The complete image of the fingerprint could be reconstituted by a specific processing system.
The reduction of the size of the sensor and hence its surface area will have the consequence of providing a major reduction of its cost of manufacture.
The invention proposes a sensor belonging to the fingerprint-reading means wherein the surface area of the sensor is smaller than the surface area of the fingerprint and delivers only partial images of the complete fingerprint. The reconstruction of the complete image of the fingerprint is obtained by the superimposition of successive images given by the sensor during its relative shift with respect to the finger.